Mother is in a foster home and she wants me to get a gift, but what?

What does this couple like to do, to read? Find out what their interests are, and look for gifts in that category. If he's a DIY'er, get him a gift card to HD or Lowe's. If she likes to cook, perhaps a gift card to a reasonably priced cooking business.

Or is there something they can use for their home?

Cash, one of the best gifts.

A cookie platter is always well-received

I thought foster homes were for children. How does a senior get a foster home? Is this something new?

And box of Sees Candies is my go-to gift when I buy for someone I don't know that well and working with a budget. Also, we have a small chain of high-end movie theaters here - you can order real food, wine, micro-brews etc. I often give gift certs from there if it's a couple that likely wouldn't/couldn't spend extra for a nice evening out - still a reasonable cost for me. I guess I usually try to aim for some sort of treat that a person isn't likely to buy for themselves.